Thermal actuators



Feb. 7, 1967 F. P. MIHM 3,302,391

THERMAL ACTUATORS Filed Jan. 25, 1965 INVENTQR FERD P. MlHM hp Magma@ATR N EYS United States Patent O 3,302,391 THEAL A TUA'IDRS Ferd I.Mihm, Needham, Mass., assignor to Tempstat Corporation, Hinsdale, NJH.,a corporation of Dela- Ware Filed Jan. 25, 1965, Ser. No. 427,664 I()Claims. (Cl. 60-23) The present invention relates to improvements inthermal actuators, and, in one particular aspect, to novel and improvedpiston-cylinder units of economical construction which respondaccurately to temperatureinduced dimensional variations in lill materialand which uniquely maintain fluid-tight sealing between relativelymovable parts.

Various forms of thermal actuators have been evolved in the past tolconvert thermal energy into mechanical forces and movements whichsupply motive power for operation of valves, switches, and the like. Oneconventional arrangement of this general type includes a rigidheat-conducting container within which is disposed a till of material,such as wax, which undergoes a marked change in rate of expansion andcontraction as the result of transitions between .solid and liquidphases within a predetermined narrow range of temperatures. Intranslating the thermally-induced dimensional variations of the tillinto related mechanical strokes of a movable output member, it has beena common practice to utilize elastomeric sleeve-like diaphragms whichmay be tightly sealed about their outer peripheries to insure that noneof the critical ll volume can be disturbed by even minute leakages.Where substantial Voutput strokes are required, such diaphragms arehighly susceptible to distortion and damage by excessive stretching,rictional abrasion, and pinching or bunching effects. Moreover, in somedesigns the resiliencies of such seals can cause the output responses tobe sluggish and can introduce hysteresis effects which prevent theoutput member from moving exactly to the intended positions as it isrepeatedly cycled in its movements. Although piston-cylinder arrays arewell suited to the type of mechanical translations which are ofinterest, the sealing problem has not heretofore been resolvedsatisfactorily, and, moreover, the expected need for precision machiningof bores, pistons, rings, and the like would introduce high costs initems which should be of inexpensive construction.

In accordance with the present teachings, the aforesaid diculties may beavoided through use of unique piston-cylinder type thermal actuatorunits wherein a special form of dynamically-operated movable elastomericpiston and seal unit cooperates with a tubular cylinder unit in which isdisposed an expansible lill.

It is one of the objects of the present invention, therefore, to providenovel and improved high-precision thermal actuators lending themselvesto low-cost manufacture and to reliable non-leaking operation withimproved hysteresis characteristics.

Another object is to provide unique thermal actuators of thepiston-cylinder type wherein movable piston units involving resilientsealing provisions are dynamically expanded to preserve uid-tightrelationships with cooperating tubular cylinders under severe operatingconditions and despite wear.

A further object is to provide expansible piston units, particularly foruse with thermal actuators, which maintain improved sealing in relationto relatively imprecise cylinder bore surfaces while undergoingreciprocating movements in dilerent directions.

Still further, it is an object to provide improved piston units whichmay be conveniently and precisely oriented and sealed within filledthermal acuator tubes to regulate accurately the responses of thethermal actuators.

3,302,391 Patented Feb. 7, 1967 ICC It is also an object to provide anovel and improved method for assembling and adjusting the responses ofthermal actuators having a fill Which is at least in part liquid.

By way of a summary account of practice of this invention in one of itsaspects, a thermal actuator, including an elongated tubular memberclosed at one end and filled at least in part with athermally-responsive material which expands markedly upon being melted,is provided with a unique movable piston serving to en close and sealthe lill near the open end of the tube. The piston comprises an assemblyof a hollow cylindrical elastomeric member, which is normally ofexternal diameter not appreciably greater than the internal diameter ofthe cooperating tube, and a pair of stiff washer-like elements disposedone at each axial end of the elastomeric member, and a central fastenerpin extending through the washers and elastomeric member, making a tightseal with the latter, and a spring supported between one end of the pinand one of the washers exerting an axial compressive force which causesthe elastomeric member to bulge circumferentially into tight sealingrelationship with the bore of the tube. An actuator stem, one end ofwhich separably engages the piston assembly, extends outwardly beyondthe open end of the tube to a spring-biased movable valve member, or thelike, and is caused to follow the movements of the piston assembly.

Although the aspects of this invention which are believed to be novelare set forth in the appended claims, additional details as to preferredpractices and as to the further advantages, objects and features of theinvention may be most readily comprehended through reference to thefollowing discription taken in connection with the accompanying drawingswherein:

FIGURE l is a partly pictorial and partly cross-sectioned side View of arelief valve arrangement wherein teachings of this invention arepracticed;

FIGURE 2 provides an enlarged illustration lof an expansible piston-sealunit `such as that used in the relief valve assembly shown in FIGURE l;

FIGURE 3 depicts the piston-seal structure `of FIG- URE 2 in a relaxe-duncompressed state prior to lockin-g of the parts together;

FIGURE 4 illustrates an improved piston-seal unit being inserted into alled actuator tube;

FIGURE 5 provides an enlarged detail of cooperating portions ot a valvestem, piston-seal unit, and thermal actuator tube, with certain parts'being cut away to disclose constructional details; and

FIGURE 6 is a partial cross-section of an alternative embodiment ofpiston-seal assembly.

A typical application for the improved thermal actuator assembly ischaracterized 'by the automatic relief valve apparatus '7 shown inFIGURE 1, wherein a normallyclosed valve unit 8 is responsive toactuating vforces developed by a thermal lsensing and actuating unit 9.Such apparatus may, for example, be installed atop a hot water tank,-via the threaded coupling 10, with the elongated sensing unit immersedin the enclosed hot water. Movable valve ymember lll, which is loadedinto tight seating and sealing engagement with a valve seat 12 withinthe valve vbody 13 by a compressed spring 14, normally prevents escapeof the tank contents through inlet coupling It? and outlet passagewayI5. The seating force developed by spring 14, between t-he valve imember11 and an adjustable valve body cap I6, is set at the factory to insurethat the valve is opened by tank pressure only when it falls within apredetermined narrow range of pressures. This same valving member isalso .separately actuatable manually, `by an external camming handle 17which is pinned to a slidable shaft or stem 13 sealed in relation to cap16 and xed with member 11. The aforesaid automatic and manual pressurerelief provisions are in `addition to those for `automatic valiving inresponse to stem movements developed by the thermal actuator unit 9.

For the latter purposes, an elongated tubular member 19 has its open end20 fixed with a radially-spoked valve body bracket 21, through whichfluid may flow, and its closed end 22 extends into the tank liquid foroptimum response to its temperatures. Tube 19 is formed with asubstantially even inner surface 23 of substantially one diameter, andis of material exhibiting good thermal conductivity characteristics andresistance to corrosion. The portion of valve stem 18 which projectsdownwardly from valving member 11 extends into the open end of tube 19and engages a special piston-seal unit 24 mated within that tube.Preferably, the stem and piston-seal funit are separable from oneanother, axially, although they are maintained in engagement by thecombined actions of load spring 14 and of a fill 25 ofthermally-responsive material which occupies the space betweenpiston-seal unit 24 and the closed end 22 of the tube. According to onecommon practice, the fill may consist essentially of a wax which has amelting point at about the critical temperature at which the valvingmember 11 should be lifted from its seat t-o `open the pr-otectiverelief valve (such as a temperature just below the boiling point of thewater in the tank). Petroleum-type waxes having widely different meltingtemperatures are readily obtainable commercially, for example, and maybe blended to achieve desired melting characteristics. As is wellunderstood in the art, certain waxes and like substances exhibit a verysubstantial increase in volume when they make the transition from asolid to liquid state within a predetermined narrow temperature range,and this increase will cause the piston-seal unit 24 to move, raise thestem 18, and lift the valvin-g member 11 4from its seat, with adesirably swift and :positive action for relief valving purposes.

In the past, the tendencies for the pressurized liquid (melted) wax orlike material to leak past movable seals has necessitated use offlexible diaphragms and sleeves, generally of elastomeric material,which could be clamped or otherwise bonded about their peripheries toinsure that none of the liquid could escape and thereby seriously alterthe fill volume and resulting operating characteristics of the actuator.Such deformable elastomeric seals tend to involve undesirably highcosts, are particularly susceptible to damage, and are difficult toorient in relation to a particular volume of fill which will reliablyprovide the precise response sought. By way of distinction, thepiston-seal unit 24 may be reciprocated without significant loss offill, so effectively in yfact that the tube may conveniently andadvantageously include a fill not only of normally solidified wax butalso of a supplemental `forcetransmitting material, such as water, whichis normally in the liquid state and would be expected to pose veryserious leakage problems. As is shown in detail in FIG- URES 2 and 3,the piston-seal `unit is a separate integrated structure including asits principal components a tubular elastomeric resilient member 26, apair of rigid washers 27 `and 28 one :at each axial end of member 2d, ahelical loa-ding spring 29, and a central headed fastening pin member3f). The length 31, external diameter 32, and internal diameter, ofresilient member 26 in the relaxed or uncompressed state (FIGURE 3) areselected such that the member will, when compressed axially (FIGURE 2)to a shorter axial length 33, tend to bulge outwardly to a materiallygreater external diameter 34, while at the same time remaining in tightsurrounding relationship with the central fastener 30. For the latterpurposes, the axial forces exerted on member 26, when contained -by theinternal diameter of the tube, produce radial pressure-sealing `forcesboth on the walls of the d internal diameter of the tube and on theouter diameter of the pin 3f). In addition, the inner diameter of theresilient member 26 may be made initially somewhat less than the outerdiameter 35 of pin 35i, with the pointed end 3fm of the lfastenerfacilitating the initial assembly of the differently-sized parts. Theexpanded diameter 34 is selected to be materially in excess of the innerdia-meter of the cooperating actuator tube 19, such that resilientmember 2d will constantly maintain Huid-tight sealing with the tubeunder all expected operating conditions. Before being inserted into theactuator tube, the relaxed loose assembly (FIGURE 3) is compressedaxially by the desired `amount which will tend to maintain the aforesaidtight sealing, and that compressed condition is preserved by a crimping30]) below the level of washer 27. The washers 27 and 28 lprevent damageto the resilient member 26 by distributing the axial compression forcesuniformly, and these are of just slightly smaller diameter than that ofthe actuator tube interior. Spring 29 and the enlarged head 30C offastener 3f) are preferably enough smaller in diameter than the tubeinterior to accommodate the lower end of hollow stem 18 in asurroundin-g loosely-nested relationship therewith. As appears in FIGURE1, the expanded resilient piston-seal member 24 engages the inner sidewalls of tube 19 over a major portion of its length, rather than merelyat the crest of the bulge shown in FIGURE 2, the dimensional changebeing accommodated by increased longitudinal compression of load spring29.

An important aspect of the self-loaded piston-seal unit is recognized byconsidering the potentially troublesome effects which could be present,absenting spring 29, when the fill in an actuator tube contracts involume but the spring loading by the main valve spring 14 does not actconcomitantly to drive the stem 1S against the washer 28 forcefullyenough to compress resilient member 26 into the needed ti-ght sealingengagement with the interior of tube 19. This condition could developwhen either the internal tank press-ure -or the manually-operated lever17 causes the valve member 11 to be raised slightly `from its seat. Theresulting negative pressure effects on resilient member 26 would thentend to draw compressible air or vapor from the tank into the volumebelow the member 26, and, thereafter, the effective ll volume would bedisturbed, as would its effective incompressibility also, and theactuator would thus fail to operate with the intended characteristics.However, in the improved dynamically-operating unit 24, including theself-loading spring 29, the latter spring is effective to maintain atleast such axial compression of member 2e as Will suppress such reverseleakage even when the lseparable stem 18 is widely separated `from thewasher 28. This self-loading spring also causes the unit to beself-compensating `for effects of frictional wear. Forces required toreturn the piston-seal unit to a predetermined position within the tube,after forced displacement therefrom and after cooling of the fill, arerelatively low and predictable, such that the hysteresis characteristicsof the actuator are highly desirable.

Further advantages afforded by the selfloaded unit 24 are realized whilethat unit is being inserted into the tube 19 (FIGURE 4). -It isimportant that the exterior surfaces -of resilient member 26 be smoothlyapplied against the cooperating inner .surfaces of the tube, and thatsevere pinching or other -distortions and damage be avoided. The-oversize member 26 would be somewhat vulnerable to these disturbancesif the mating forces were applied directly to washer 28. However, it isfound that the mating operation is improved by applying forces in thedirection of 4arrow 36 to the fastener head 30C by way of a tool 37,instead. Under these conditions, any excessive forces on the resilientmember 26 will result in relieving longitudinal expansion against therestraint of the load spring 29, and the resilient member will slip intoposition more readily and smoothly. These improvements are of particularsigniiicance when the tube 19 is filled (FIG- URE 4) in part by asolidified wax 25a, or the like, and in part by a force-transmittingincompressible liquid h, such as water, which occupies the remainingvolume within the tube. Some of the liquid must be permitted to escapeduring the insertion step, to permit the piston-unit t-o reach apredetermined mated position, and the axial resilient relief afforded byspring 29 advantageously accommodates the needed by-pass leakage 3S -atthe open end of the tube. This practice involving the supplementalliquid 4lill permits a highly precise setting of the actuator to bemade, and at room temperatures, whereas, if the Same technique is soughtto be used with a wax ll alone, the wax must first be melted, thetemperatures must be high, the clean-up released wax is troublesome, andit is extremely diflicult to control the exact critical volume of waxtill which will be present in the completed actuator. The liquid usedmust be substantially noncompressible, should be immiscible in the waxor other lill material, should exist in the liquid state at roomtemperatures and at the higher temperatures at which the wax or otherlill material melts, and, preferably, should have about the same thermalexpansion characteristics as the solidified till material. Theserequirements can be adequately satisfied by substantially pure waterused with a solidified wax Vtill material; one such wax fill undergoesabout a ten percent change in volume upon melting, whereas the waterundergoes only a relatively small change in volume within the samenarrow range of temperatures. The inexpensive incompressible liquidsupplemental fill material serves to transmit the forces of wax lillexpansions to the piston-seal unit, and thereby permits only arelatively small amount of the more costly wax to be located at a remotesensing site for purposes of driving the piston-seal unit locatedelsewhere; flexible tubing may intercouple the two sites, and, in anyevent, the output stem -or other member driven by the pistonseal untilneed fbe only relatively short and costly metal may thus be conserved.Furthermore, even where only a relatively small amount of the liquid isused, it serves to overcome the problem associated with voids appea-ringbetween the solidified wax fill and the piston-seal unit. In thisconnection, FIGURE 4 discloses the type of solidication vortex 39 whichcommonly results from wax soliditication, and it is evident that thisdepression will tend -to establsh a void, between the wax Vlill andpiston-seal unit, which would tend to adversely aiiect the operatingcharacteristics of the thermal actuator. Liquid 25b eliminates thisadverse iniiuence. Another unobvious advantage accrues when as littlewax as possible is Iused, only to develop the minimum output movementrequired for any application, with a relatively large amount of liquidlill being used at the same time. By way of explanation, it has beenfound that wax fill material alone tends to exhibit an undesirablespringiness or compressibility, which renders the operatingcharacteristics of the actuator less predictable. When a relativelylarge quantity of a more incompressible liquid, such as water, is addedin the actuator cavity, the total springiness or compressibility issignificantly and advantageously reduced.

The detail appearing in FIGURE 5 discloses the cooperation between thepiston-seal unit 24 and a hollowed separable output member or tubularstem 18a, which is generally like stern 18 in FIGURE l except that itsouter diameter is large enough to permit a sliding fit within tube 19which will guide the stem more accurately and better avoid anylikelihood that the piston-seal unit will be cocked to one side by thestem. The length over which the stem and upper portion of thepiston-seal unit are slidably mated is adequate to insure that they willnot become entirely separated if the piston-seal unit does not followthe stem movements at all times, such as times when excess pressure ormanual operation of the valve cause the valve stem to be liftedindependently.

Preferably, the compressed axial length of the resilient member(substantially length 33 of resilient member 26, in FIGURE 2) issufficient to prevent the piston-seal unit from tipping out of itsintended concentric alignment with the surrounding tubular actuatormember. However, in an alternative construction such as that illustratedin FIGURE 6, the resilient member, 26', may be of a relatively shortlength, with one of the end washers being replaced by an elongatedhollow cylindrical element, 28', which is of outer diameter justslightly smaller than the inner diameter of the actuator tube 19. Loadspring 29 and the central fastener 30 function substantially in lthemanner earlier described in connection with the unit of FIGURE 2, topreserve good sealing at all times, while the element 28 guides the unit24 to prevent misalignment of member 26 in tube T19.

The resilient piston-seal members are formed of silicone rubber, orsynthetic materials or rubber which is not aiected by hydrocarbons (suchas those of a wax fill) and does not contain sulphur or other materialswhich would be likely to cause undesirable adherence to the actuatortube (such as a brass tube) at high temperatures. Where a high degree oflubricity is desired between the actuator tube and resilientpiston-seal, a very thin and flexible sleeve of a relatively slipperytetraiiuoroethylene material may be disposed around the periphery of theresilient member, or the friction may be reduced by tumbling thecooperating parts in mica or by subjecting the resilient member to asurface treatment such as the so-called LCR treatment furnished by J.Royal Company, Barrington, Rhode Island, or by applying to thecooperating parts a uorocarbon such as that sold by Du Pont Corporationunder the trade name Vydaxar. Although a helical load spring has beenshown in association with the piston-seal units, other forms ofresilient loading elements may be substituted, and, in some instances,the end of the piston-seal unit which carries the load spring may bereversed in position (i.e. may project into the fill, rather than extendtoward the stem). Actuator units expressing these teachings may ofcourse be used for purposes other than the specific valve actuationsdiscussed herein.

Accordingly, although preferred. practices and embodiments of thisinvention have been shown and described, it should be understood thatvarious modifications, additions and substitutions may be effected bythose skilled in the art without departure from these teachings, and itis aimed in the Aappended claims to embrace all such variations as fallwithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

ll. A thermal actuator comprising a closed container having innersurfaces defining a rigid open-ended cylinder accommodatingreciprocations of a piston unit therein, thermally-responsive tillmaterial within said container exhibiting significanttemperature-induced dimensional variations within a predeterminedtemperature range, and an elongated piston-seal unit disposed withinsaid cylinder and movable along the longitudinal axis thereof, saidpiston-seal unit including a piston member of resilient material havinga cross-section normally not in excess of the cross-section of saidcylinder, elongated fastening means extending through said member in thedirection of said axis, and resilient loading means mounted on andmovable as a whole with said fastening means in a forcetransmittingrelation to said piston member, said fastening means includingaxially-spaced holding means movable therewith axially compressing saidpiston member and resilient loading means together between said holdingmeans, whereby the cross-section of said axially-compressed pistonmember tends to be greater than that of said cylinder and iiuid leakagefrom and into said container is suppressed by said piston-seal unit.

2. A thermal actuator as set forth in claim ll, wherein said cylinderhas a substantially circular and uniform cross-section, wherein saidpiston member is a normally substantially cylindrical hollow member ofelastomeric material, wherein said resilient loading means comprises aspring, wherein said fastening means comprises an elongated memberextending axially through said piston member and tightly sealedtherewith, and wherein said holding means comprises enlargements at bothends of said elongated member preventing separation thereof from saidpiston member and compressing said spring between one of saidenlargements and one side of said piston member.

3. A thermal actuator as set forth in claim 2 wherein said movablepiston-seal unit further includes means distributing compression forcesfrom said spring substantially uniformly over the ends of said pistonmember, at least one of said distributing means comprising asubstantially rigid washer of 4diameter slightly less than that of saidcylinder and mated with said elongated member, and wherein said springcomprises a helical spring disposed coaxially around said elongatedmember between said one of said enlargements and said washer.

4. A thermal actuator as set forth in claim 1 further including amovable output member disposed at least in part outside of saidcontainer and responsive to movements of said piston-seal unit withinsaid cylinder, and means connecting one end of said piston member inforcetransmitting relation to said output member independently of saidresilient loading means.

5.A thermal actuator comprising an elongated rigid heat-conducting tubeclosed at one end and having inner surfaces near the opposite endthereof defining a -cylinder of substantially circular and uniformcross-section, thermally-responsive fill material within said tubeexhibiting significant temperature-induced changes in volume within apredetermined temperature range, an elongated piston-seal unit disposedwithin said cylinder, said pistonseal unit including a normallysubstantially cylindrical hollow resilient member of elastomericmaterial having a cross-section normally not in excess of that of saidcylinder, a helical loading spring, an elongated fastening memberextending axially through said resilient member and tightly sealedtherewith, said fastening member having enlargements at both endspreventing separation thereof from said resilient member, and meansdistributing compression forces from said loading spring substantiallyuniformly over the ends of said resilient member, said helical springbeing disposed coaxially around said elongated fastening member betweenone end of said fastening member and said force-distributing means,whereby said fastening member fastens said spring and resilient membertogether for reciprocating movements Itogether within said cylinder andwith said spring compressing said resilient member and tending toincrease the cross-section thereof to a cross-section greater than thatof said cylinder, and whereby fluid leakage from and into said containeris suppressed by said movable pistonseal unit, and means outside of saidtube responsive to movements of said piston-seal unit within saidcylinder and including a movable output member having a hollow end insurrounding relation to said loading spring and said one end of saidfastening member and separably engaging said force-distributing means,said output member extending outside of said tube, and resilient meansoutside of said tube urging said end of said output member intoengagement with said force-distributing means.

6. A thermal actuator as set forth in claim 5 wherein saidthermally-responsive fill material includes normallysolidified Waxwithin said tube near the closed end thereof and a supplement fill olfliquid in which the wax is immiscible occupying the space between thewax and said piston-seal unit.

7. A thermal actuator comprising an elongated heatconductive cylindricaltube closed at one end and open at the other end, an elongated movablepiston-seal unit disposed within said tube near said open end, saidmovable piston-seal unit including an axially-compressible cylindricalresilient member coaxially surrounding an elongated central fastenerextending therethrough and axially compressing said member together witha load spring sufiiciently to expand said member into iiuid-tightsealing relationship with said tube, a normally-solidified wax withinsaid tube near the closed end thereof, water filling the space betweensaid Wax and piston-seal unit within said tube, and a movable outputmember in force-transmitting relationship to one end of said resilientmember independently of said load spring.

8. A thermal actuator as set forth in claim 7 wherein said load springcomprises a helical spring coaxial with said fastener on the side ofsaid resilient member nearer said open end of said tube, and wherein atleast one end of said output member is hollow and in surroundingrelationship to said spring and is axially movable in relation to saidresilient member for a distance not in excess of the length of saidspring.

9. The method of assembling a thermal actuator having a container whichis to be closed by a movable pistonseal unit having anaxially-compressible resilient member radially expansible intofluid-tight sealing engagement with the container by a load springmovable therewith, which comprises partly filling the container with apredetermined amount of a normally-solidified thermallyresponsivematerial, filling the remainder of the container with aforce-transmitting material which is liquid at room temperatures and atthe melting temperature of said normally-solidified material, at roomtemperature applying mating forces to one end of the resilient member indirection to mate the resilient member with the container whilepermitting the resilient member to elongate axially in directionopposite to that of compressixe forces exerted by the load spring andthereby permit by-pass leakage of the liquid until the piston-seal unitis in a predetermined mated position within the container, andthereafter directly applying forces from the one end of the resilientmember to an output member While maintaining restraint of the loadspring on the resilient member to expand it radially into duid-tightsealing engagement with the container.

10. The method of assembling a thermal actuator as set forth in claim 9wherein the steps of filling the container comprise partly filling thecontainer with a predetermined amount of heated wax in liquid form,permitting the heated wax to solidify, and filling the remainder of thecontainer with water, and thereafter applying the mating forces to matethe piston-seal unit with the container with attendant by-pass leakageof excess water.

References Cited by the Examiner UNITED STATES PATENTS 2,938,384 5/1960Soreng et al. 73-368-3 X 3,007,029 10/1961 Levine 73-368.3 X 3,180,1504/1965 Horne 73-368-1 3,194,009 7/1965 Baker 60--23 EDGAR W. GEOGHEGAN,Primary Examiner.

1. A THERMAL ACTUATOR COMPRISING A CLOSED CONTAINER HAVING INNERSURFACES DEFINING A RIGID OPEN-ENDED CYLINDER ACCOMMODATINGRECIPROCATIONS OF A PISTON UNIT THEREIN, THERMALLY-RESPONSIVE FILLMATERIAL WITHIN SAID CONTAINER EXHIBITING SIGNIFICANTTEMPERATURE-INDUCED DIMENSIONAL VARIATIONS WITHIN A PREDETERMINEDTEMPERATURE RANGE, AND AN ELONGATED PISTON-SEAL UNIT DISPOSED WITHINSAID CYLINDER AND MOVABLE ALONG THE LONGITUDINAL AXIS THEREOF, SAIDPISTON-SEAL UNIT INCLUDING A PISTON MEMBER OF RESILIENT MATERIAL HAVINGA CROSS-SECTION NORMALLY NOT IN EXCESS OF THE CROSS-SECTION OF SAIDCYLINDER, ELONGATED FASTENING MEANS EXTENDING THROUGH SAID MEMBER IN THEDIRECTION OF SAID AXIS, AND RESILIENT LOADING MEANS MOUNTED ON ANDMOVABLE AS A WHOLE WITH SAID FASTENING MEANS IN A FORCETRANSMITTINGRELATION TO SAID PISTON MEMBER, SAID FASTENING MEANS INCLUDINGAXIALLY-SPACED HOLDING MEANS MOVABLE THEREWITH AXIALLY COMPRESSING SAIDPISTON MEMBER AND RESILIENT LOADING MEANS TOGETHER BETWEEN SAID HOLDINGMEANS, WHEREBY THE CROSS-SECTION OF SAID AXIALLY-COMPRESSED PISTONMEMBER TENDS TO BE GREATER THAN THAT OF SAID CYLINDER AND FLUID LEAKAGEFROM AND INTO SAID CONTAINER IS SUPPRESSED BY SAID PISTON-SEAL UNIT.